The electrolysis industry represented by chlor-alkali electrolysis plays an important role as a material industry. Although the industry has such an important role, chlor-alkali electrolysis consumes a large quantity of energy. Further in this regard, energy savings is a priority in countries where the cost of energy is high, as in Japan. For example, the shift in chlor-alkali electrolysis from a mercury process using a diaphragm to an ion-exchange membrane process for the purpose of eliminating environmental problems and simultaneously attaining energy saving has attained an energy savings of about 40% over a period of about 25 years. However, even this energy savings is still insufficient, because the cost of electric power used as an energy source accounts for 50% of the total production cost. The situation has reached the stage where additional power savings cannot be attained using the current process. In order to attain further energy savings, a drastic change is necessary using, for example, electrode reactions different from conventional ones. An example thereof is the use of a gas diffusion electrode which is employed in fuel cells, etc. This is the most feasible among the currently known means and provides considerable power savings.
Gas diffusion electrodes are characterized as having the property of enabling a gas as a reactant to be easily fed to the electrode surfaces, and such electrodes have been developed for use in fuel cells, etc. Recently, investigations have begun on the utilization of gas diffusion electrodes in industrial electrolysis. For example, in an apparatus for the on-site production of hydrogen peroxide, a gas diffusion electrode has been utilized as a hydrophobic cathode for conducting an oxygen-reducing reaction (see "Industrial Electrochemistry" (2nd ed.) pp. 279-, 1991). In alkali production and various recovery processes, gas diffusion electrodes are used to conduct anodic hydrogen oxidation or cathodic oxygen reduction. This takes the place of oxygen generation at the anode or hydrogen generation at the cathode as a counter-electrode reaction so as to diminish power consumption. It has been reported that the use of a hydrogen anode as a counter electrode in metal recovery, e.g., zinc collection, or in zinc plating is effective in attaining depolarization.
However, these industrial electrolytic systems are disadvantageous in that the electrode does not have sufficient operating life or sufficient performance. This is because the composition of the solution or gas or the operating conditions are complex as compared with the case of fuel cells.
An example of a process for producing sodium hydroxide by the electrolysis of sodium chloride is explained below. Sodium hydroxide and chlorine, which both are important substances for use as industrial starting materials, are produced mainly by the electrolysis of sodium chloride. As discussed above, this electrolytic process has shifted to the ion-exchange membrane process, which employs an ion-exchange membrane as a diaphragm and an activated cathode having a low overvoltage. By using an ion-exchange membrane, the electric power consumption rate of sodium hydroxide production was reduced to 2,000 kWh per ton of sodium hydroxide. When an oxygen reduction reaction not involving hydrogen generation is conducted in place of hydrogen generation at the cathode in conventional processes, the theoretical decomposition voltage decreases from 2.19 V, which is the conventional value, to 0.96 V. Namely, a decrease in theoretical decomposition voltage of 1.23 V is possible, and great energy savings is expected.
In order for this new process to be realized industrially, it is indispensable to develop an oxygen gas diffusion cathode (a gas diffusion cathode for which oxygen is used as a feed gas) having high performance and exhibiting sufficient stability in the electrolytic system described above.
FIG. 1 shows a diagrammatic view of an electrolytic cell for sodium chloride electrolysis which employs an oxygen gas diffusion cathode of the most common type that is currently used.
This electrolytic cell 1 is partitioned into an anode chamber 3 and a cathode chamber 4 with a cation-exchange membrane 2, and the cathode chamber 4 is partitioned into a solution chamber 6 and a gas chamber 7 with an oxygen gas diffusion cathode 5. Oxygen gas as a starting material is fed from the gas chamber 7 side to the gas phase side of the oxygen gas diffusion cathode 5. The oxygen gas diffuses through the oxygen gas diffusion cathode 5 and reacts with water in the catalyst layer within the cathode 5 to generate sodium hydroxide. Consequently, the cathode used in this electrolytic process should be a gas diffusion electrode of the so-called gas/liquid separation type, which is sufficiently permeable to oxygen only and prevents sodium hydroxide from moving from the solution chamber to the gas chamber through the electrode. The oxygen gas diffusion cathodes which have been proposed so far as electrodes for sodium chloride electrolysis satisfying the above requirement are mostly gas diffusion electrodes produced by mixing carbon powder with PTFE, molding the mixture into a sheet to obtain an electrode base, and depositing a catalyst, e.g., silver or platinum, on the base.
In conventional sodium chloride electrolysis, the anodic and cathodic reactions are as follows, and the theoretical decomposition voltage is 2.19 V.
Anodic reaction: 2Cl.sup.- .fwdarw.Cl.sub.2 +2e (1.36 V)
Cathodic reaction: 2H.sub.2 O+2e.fwdarw.4OH.sup.- +H.sub.2 (-0.83 V)
When the above electrolysis is conducted while feeding oxygen to the cathode, hydrogen is consumed by the oxygen supplied to the electrolytic cell, resulting in the following cathodic reaction.
Cathodic reaction: 2H.sub.2 O+O.sub.2 +4e.fwdarw.4OH.sup.- (0.40 V)
Therefore, a power consumption reduction of 1.23 V is theoretically possible and, even in a practical current density range, a reduction of about 0.8 V is possible. Namely, a power savings of 700 kWh per ton of sodium hydroxide is theoretically attainable. Although investigations on the practical use of gas diffusion electrodes for sodium chloride electrolysis have been made since the 1980's from the standpoint of such energy saving, these type of electrodes have the following drawbacks.
(1) The carbon used as an electrode material readily deteriorates at high temperatures in the presence of both sodium hydroxide and oxygen to considerably impair electrode performance.
(2) With an increase in liquid pressure and with electrode deterioration, it becomes difficult to prevent the sodium hydroxide thus generated from leaking into the gas chamber.
(3) It is difficult to fabricate an electrode having a size necessary for practical use (1 mm.sup.2 or larger).
(4) Although the pressure within the cell changes with height, it is difficult to obtain a pressure distribution of oxygen gas that is supplied to the electrolytic cell which compensates for the pressure change.
(5) There is a solution resistance loss due to the catholyte, and power for stirring the solution is necessary.
(6) For practical use of the electrode, existing electrolytic facilities must be modified considerably.
(7) If air is utilized as an oxygen-containing gas, the gas-diffusing ability of the electrode is reduced. This is because carbon dioxide contained in the air reacts with sodium hydroxide to deposit sodium carbonate on the walls of the pores of the gas diffusion electrode.
An electrolytic process which eliminates these problems is the zero-gap electrolytic process which employs the electrolytic cell shown in FIG. 2. This electrolytic process is characterized in that the electrolytic cell 8 has an oxygen gas diffusion cathode 9 and an ion-exchange membrane 10 which are in intimate contact with each other to thereby omit the solution chamber shown in FIG. 1. Oxygen gas and water are fed as starting materials, and sodium hydroxide as a reaction product is recovered from the same side.
This electrolytic process is free from gas leakage from the solution chamber into the gas chamber. Hence, the above problem (2) is eliminated. Furthermore, because the electrolytic cell has a structure such that the electrode is in intimate contact with the ion-exchange membrane, electrolytic facilities for the conventional ion-exchange membrane process can be used without necessitating considerable modifications. Hence, the above problems (5) and (6) are also eliminated.
The performance criteria required of oxygen gas diffusion cathodes suitable for use in this electrolytic process are as follows: high gas permeability; high hydrophobicity which is necessary to avoid wetting by sodium hydroxide; and high permeability required for sodium hydroxide to move within the electrode. For attaining these requirements, the oxygen gas diffusion cathode described above is made of a durable metal, e.g., nickel or silver. Hence, the above problem (1) is eliminated and long-term electrolysis can be expected.
Furthermore, because the sodium hydroxide that is recovered in this electrolytic process permeates through the cathode into the oxygen feed side, partitioning into a solution chamber and a gas chamber with a cathode as in conventional processes is unnecessary. Consequently, no problem arises even when liquid permeates through the electrode, and electrode enlargement is thought to be relatively easy to thereby eliminate the problem (3). Because the electrolytic cell has no solution chamber and hence undergoes no liquid pressure change in the height direction, the cell is, of course, free from the problem (4). In addition, because the sodium hydroxide thus produced necessarily moves through the electrode to the oxygen feed side, the problem (7) is less apt to occur.
As described above, attempts have been intermittently made to apply gas diffusion electrodes to industrial electrolytic systems, and these attempts have succeeded in making various improvements and in providing desirable results. However, in the case where an existing electrolytic cell having a height as large as 1 m is to be utilized, even a gas diffusion electrode having the structure described above cannot provide its intrinsic electrolytic performance. Namely, gas feeding is inhibited. This is because the alkali solution which is moving toward the oxygen feed side and also the liquid which has moved gravitationally in the height direction resides within the electrode.